I happened to read Steve Stroud's recent posting on Red Star ale yeast
just as I was preparing a batch of YPD plates, so I decided to plate
some and have a look for myself. I suspended about 1/4 tsp of the yeast
in a little cold water, let it rehydrate 10 min, and streaked for single
colonies.

The result is that out of about 1000 single colonies I found three
unmistakable bacterial ones, one doubtful, and the rest clearly yeasts.
Bacterial contamination does not thus appear terribly large (at least
for aerobic and facultative-aerobic bacteria able to grow on YPD). On
the other hand, of the yeast colonies some 40% were very small,
presumably "petite" respiratory mutants, although I cannot exclude that
some are slow-growing non-saccharomyces.

I picked a couple of healthy, vigorous colonies, restreaked them, and so
far they appear to breed true, without new petite mutants.